With the increased usage of modern structures and structural systems, industry is faced with even more demands to understand performance during usage and durability over time. This situation occurs on metallic, composite and ceramic structures where the presence of damages could affect their structural integrity. This situation also occurs where changes in chemical atmosphere or surrounding environment due to leakages are critical for the performance of a system or for the health of personnel or ecosystem. In many cases, highly stressed areas or “hot spots” occur on zones with limited access or no access at all. These hot spots can be found on structural elements that transfer load from one sub-structure system into another. For example, in aircraft, hot spots are frequently found on structural interface members such as landing gear/wing, landing gear/fuselage, wing/fuselage, engine pylon/fuselage, and bulkheads/fuselage among others. Some of those structures are critical and may hide serious damage. Current Non Destructive Inspection (NDI) methods detect and monitor damage when a structure is undergoing routine maintenance. However, inspecting assembled structures is difficult and sometimes impossible due to hardware assembly and/or limited access. In addition, conventional inspections of these spots may require time consuming and expensive disassembly tasks.
Thanks to the advance of embedded systems, additive manufacturing, and signal processing, there is a transition from conventional NDI methods into Structural Health Monitoring (SHM) methods. Placing sensors on the structures has been around for more than 30 years and it has demonstrated feasibility in laboratory and controlled testing environments on aerospace, mechanical engineering, and civil infrastructure applications. See Farrar C., and Worden, K., “An Introduction to Structural Health Monitoring,” Phil. Trans. R. Soc. A (2007), 365. In general, SHM consists of a set of sensors and actuators embedded into a given system or bonded onto one of its surfaces such as plates, blades, or panels, plus electronics for interrogation, data acquisition, and processing. It is important to mention that conventional sensors do not measure any damage, they only sense the structure response to a given excitation signal or change in environmental conditions. Thus, differences between responses measured at different times or above some given threshold may indicate the presence of damage or a change of physical or chemical state.